SIN (SiNx, silicon nitride) thin films, characterized by impressive performances such as high dielectric constant, high insulation strength, low electric leakage, etc, are widely applied in microelectronic process for passivation, isolation, capacitance dielectric, etc. In addition, SIN films are also superior in terms of mechanical performance and stability.
The stress of SIN thin film is usually controlled within 100˜200 MPa. In deposition of SIN thin films in a microelectronic mechanical system (MEMS) or other special technical process, SIN thin film with lower stress, such as 0±50 MPa, are required.
Most SIN films produced by a process of chemical vapor deposition (CVD) have the problem of relatively high mechanical stress, especially those produced by a process of low pressure chemical vapor deposition (LPCVD), whereby the SIN film can only be deposited up to 300 nm in thickness and may fracture or even drop off when the thickness is beyond 300 nm.
An SIN thin film produced by a process of Plasma-enhanced chemical vapor deposition (PECVD) is in a better situation in terms of stress than that produced by LPCVD, but it is subject to major influence by various processing conditions. The influencing factors include temperature, gas flow ratio, reaction pressure, etc.
The traditional method for eliminating stress of an SIN film produced by the PECVD process is to employ two power sources with different frequencies to eliminate stress difference. The high frequency source has a frequency of scores of megahertz (MHz), and the low frequency source has a frequency ranging from scores to hundreds of kilohertz (KHz). Since low-frequency plasma generates compressive stress, and high-frequency plasma generates tensile stress, the compressive stress and the tensile stress may be mutually offset by adjusting the power ratio of the high frequency source to the low frequency source to enable the two power sources to operate alternatively, thereby reducing or eliminating stress.
However, for some machines, it is impossible to successively reduce stress by simply adjusting the power ratio of the high frequency source to the low frequency source while some other processing conditions have to meet specific requirements. That is, however the power ratio of the high frequency source to the low frequency source is adjusted, it is impossible to reduce the stress to a level as specifically required.
In particular, the deposition of an SIN thin film on a NOVELLUS C1 machine usually needs to be carried out at a low temperature (below 350° C.). In such situation, it is no longer sufficient to meet the requirement by simply adjusting the power of the high frequency power source and the low frequency power source.